1. Field of the Invention
The present invention relates to video signal processing, and more particularly to a system and method for processing video signals transmitted among a plurality of users during a video conference.
2. Background of the Related Art
Various multimedia services have been provided through a mobile communication terminal, and recently users have taken an interest in video communication services.
First-generation video communication services provided point-to-point service. Recently, research has focused on providing multipoint video communication services based on the first-generation video communication service. To provide multipoint video communication services, video signals must be efficiently processed from a plurality of users.
Video conference is a representative example of a multipoint video communication service. In a video conference system, plural participants exchange images and voices through their terminals.
FIG. 1 is a block diagram illustrating a general multipoint video conference system, where there are four participants. The system includes plural video conference terminals 10 each having a video camera 10a, and a MCU (multipoint control unit) 20 for processing/converting video signals from each of the video conference terminals 10 and then generating a new video signal based on each of those signals.
The MCU maintains and manages a multipoint video conference based on a H.32x terminal standard. This involves generating and transmitting a new video signal to each of the video conference terminals in a resolution appropriate to each of the video conference terminals. To perform these functions, the MCU 20 includes a video signal processing unit (not shown) for converting a video signal transmitted from each video conference terminal into a video signal format appropriate to the video conference. In FIG. 1, the dotted lines represent coded video signals transmitted from the terminals to the MCU, and the solid lines represent re-coded video signals transmitted from the MCU to the terminals.
FIG. 2 is a block diagram of a video signal processing operation performed by an MCU in accordance with the related art. As shown, the MCU includes a decoding unit 21, a down scaling unit 22, a re-coding unit 23, and a mixer 24. The decoding unit decodes a video signal received from a plurality of video conference terminals into DCT (discrete cosine transform) signals or pixel units. The down scaling unit reduces resolution of a decoded signal outputted from the decoding unit. The re-coding unit re-codes each video signal outputted from the down scaling unit 22. And, the mixer mixes video signals coded through the re-coding unit and transmits it to the video conference terminals.
Referring to FIG. 3, the decoding unit 21 includes an inverse VLC (variable length coding) unit 21a for performing inverse VLC of a coded video signal transmitted from the video conference terminal, an inverse DCT (discrete cosine transform) unit 21b for performing inverse DCT of an output signal of the inverse VLC unit 21b, and an inverse quantizing unit 21c for performing inverse quantization of an output signal of the inverse DCT unit 21b. The output signal of the inverse DCT unit 21b is output through the inverse quantizing unit 21c by pixel units.
The down scaling unit 22 reduces resolution of the video signal output from the decoding unit 21 as ½, ¼, . . . , ½n, where n is natural number not less than 1.
The re-coding unit 23 includes a quantizing unit 23a for quantizing an output signal of the down scaling unit 22, a DCT unit 23b for performing discrete cosine conversion of an output signal of the quantizing unit 23a, and a VLC unit 23c for performing variable length coding of the output signal of the DCT unit 23b. 
Operation of the related-art MCU will now be described. Initially, plural video signals input from each video conference terminal to the decoding unit 21 are decoded into a DCT domain or a pixel domain. A resolution reduction algorithm is applied to each output signal of the decoding unit 21 in the down scaling unit 22, and each output signal is re-coded in the re-coding unit 23. The video signals output from the re-coding unit 23 are mixed and the resulting signal is transmitted to the video conference terminals. In performing DCT domain processing, each video signal transmitted to the decoding unit 21 passes through the inverse DCT unit 21b. In performing pixel domain processing, each video signal received by the decoding unit 21 passes through the inverse quantizing unit 21c. 
Both the video signal decoding process through the inverse DCT unit 21a and the inverse quantizing unit 21b and the video signal coding process through the quantizing unit 23a and the DCT unit 23b include loss coding characteristics. In the related-art MCU described above, by applying decoding and coding having loss coding characteristics consecutively for video signal processing, picture quality may be deteriorated.
In addition, in the related-art MCU, by processing a video signal through the inverse DCT unit, the inverse quantizing unit, the quantizing unit and the DCT unit consecutively, video signal processing time may be delayed. More specifically, because video signal processing is performed only by the MCU, the time required for performing video signal processing may be delayed and picture quality of the processed video signal may be deteriorated.